An important function that must be provided in optical networks is wavelength routing. In particular, wavelength routing is needed at each node, in order to combine and separate different wavelength channels. Typically, the router is realized in integrated form by using a waveguide grating, and an example is shown in FIG. 1. The router in this example only includes one input waveguide and its purpose is to separate the input channels, and transmit them to different output waveguides. The waveguide grating simply transforms each input signal, intended for a particular output waveguide, into a corresponding output image produced (approximately) at the output waveguide location. However, an undesirable feature of this router is its narrow bandwidth, caused by the strong wavelength dependence of each image location produced by the grating. Because of this variation, maximum transmission to a particular output waveguide is only possible in the immediate vicinity of the center wavelength of the corresponding passband.
In order to eliminate this problem, one must use a planar arrangement of two gratings of opposite dispersions as shown previously in U.S. Pat. No. 5,488,680 which issued on Jan. 30, 1996, U.S. Pat. No. 7,010,197 B2 which issued on Mar. 7, 2006, and U.S. Pat. No. 7,283,700, which issued on Oct. 16, 2007. By this technique, each output image becomes stationary at the location of the corresponding output waveguide, thus producing a maximally flat transmission coefficient. However, an undesirable feature of this imaging arrangement is its large size. In particular, the second patent requires between the two gratings many waveguide lenses, one for each output waveguide, and therefore this design is only feasible if the number N of output waveguides is small. On the other hand, only one lens is required by the third patent, but an undesirable feature in this case is that the output waveguides are widely spaced (as shown in a following section) and this increases again the size of the router. Furthermore, in both patents, the size of the router further increases if the required width of each channel must be nearly equal to the channel spacing. Accordingly, here the above problems are substantially reduced by using a new design that has advantages of increased number of channels, reduced size, and improved performance.